1. Technical Field
The present invention relates to a piezoelectric polymer material and a method for producing the piezoelectric polymer material.
2. Related Art
As piezoelectric materials, conventionally, PZT (PBZrO3—PbTiO3-based solid solution), which is a ceramic material, has often been used. However, since PZT contains lead, piezoelectric polymer materials, which are more environmentally friendly and highly flexible, have been increasingly used.
The currently known piezoelectric polymer materials are generally classified into the following two types, i.e., poled polymers including nylon 11, polyvinyl fluoride, polyvinyl chloride, polyurea and the like, and ferroelectric polymers including polyvinylidene fluoride (β-type) (PVDF), vinylidene fluoride-trifluoroethylene copolymer (P (VDF-TrFE)) (75/25) and the like.
However, since piezoelectric polymer materials are inferior to PZT in terms of piezoelectricity, there is demand for improvement in piezoelectricity of piezoelectric polymer materials. Therefore, attempts have been made from various viewpoints in order to improve piezoelectricity of the piezoelectric polymer materials.
For example, PVDF and P(VDF-TrFE), which are ferroelectric polymers, exhibit excellent piezoelectricity among polymers and have a piezoelectric constant d31 of 20 pC/N or more. In order to impart piezoelectricity to film materials formed from PVDF or P(VDF-TrFE), polymer chains are oriented in a stretching direction by performing stretching and opposite charges are applied to both sides of the film by corona discharge or the like, whereby an electric field is generated in a direction perpendicular to the film surface and permanent dipoles, containing fluorine in side chains of the polymer chains, are oriented in a direction parallel to the electric field direction.
However, there have been problems from practical viewpoints in that opposite charges, such as water and ions in air, easily attach to the polarized film surface in a direction of canceling the orientation and, as a result, orientation of the permanent dipoles that have been arranged by the poling treatment is loosened, thereby causing a significant decrease in piezoelectricity with time.
PVDF is a material that exhibits the highest piezoelectricity among the piezoelectric polymer materials. However, since it has a comparatively high dielectric constant, which is 13, among piezoelectric polymer materials, the value of a piezoelectric g constant (open circuit voltage per unit stress), which is a value obtained by dividing a piezoelectric d constant by the dielectric constant, is small. In addition, although PVDF exhibits a favorable conversion efficiency from electricity to sound, its conversion efficiency from sound to electricity is yet to be improved.
In recent years, polymers having optical activity, such as polypeptides and polylactic acids, have gathered attention in addition to the piezoelectric polymer materials as described above. Polylactic acid-based polymers are known to exhibit piezoelectricity by simply performing mechanical stretching.
Among polymers having optical activity, piezoelectricity of polymer crystals, such as polylactic acid, results from permanent dipoles with C═O bonds that are present in a helical axis direction. In particular, polylactic acid is an ideal polymer among polymers having helical chirality due to its low volume fraction of side chains with respect to a main chain, and its large proportion of permanent dipoles per volume.
It is known that the polylactic acid, which exhibits piezoelectricity by simply performing stretching, does not require a poling treatment and its piezoelectric modulus does not decrease over the years.
Since polylactic acids exhibit various piezoelectric properties as described above, piezoelectric polymer materials produced from various types of polylactic acids have been reported.
For example, a piezoelectric polymer material that exhibits a piezoelectric modulus of approximately 10 pC/N at room temperature, which is produced by stretching a molded product of polylactic acid, has been disclosed (e.g., see Japanese Patent Application Laid-Open No. 5-152638).
It has also been reported that a high piezoelectricity of approximately 18 pC/N can be achieved by carrying out a special orientation method, which is referred to as forging, in order to make polylactic acid crystals highly oriented (e.g., see Japanese Patent Application Laid-Open No. 2005-213376).
However, both piezoelectric materials shown in Japanese Patent Application Laid-Open No. 5-152638 and Japanese Patent Application Laid-Open No. 2005-213376 are insufficient in transparency.